A spokesperson from Valencia's regional ministry of health told BBC News that food safety officers inspected RiFF on Monday but were unable to find an obvious cause.

Samples of some of the dishes that were served as part of a tasting menu have now been sent to the National Toxicology Institute for analysis, they added.

Some reports have suggested that morel mushrooms may have been to blame, but officials told local media they were waiting for the test results to come back before singling out any particular ingredient.

RiFF's owner and head chef Bernd Knöller said in a statement that he was working with health officials to establish the facts.

"Regardless of the reason that may have caused this situation, I want to convey my deep regret for what happened, hoping that soon all these facts can be clarified," he added.

"I have made the decision that the restaurant remains closed until the causes of what happened are established."